starkernel: item 3.4 -- density ranking

Punch list §25 item 3.4 complete.

stadium_density(cell_index) reads a header's heat and mass and returns
heat / mass -- a division on demand from fields already stored in the
cell, matching §19.3's "read, not computed by a scheduler" literally.
Stays valid Q48.16 without a special fixed-point routine, since heat
is already Q48.16 and mass is a plain integer divisor.

mass == 0 and an out-of-range cell_index both return 0 rather than
dividing by zero -- an empty or never-admitted slot has no footprint
to be dense within.

Deliberately not built here, per the item's own wording: finding the
densest or least-dense resident (§19.3's admission/eviction
comparison) is item 3.5's scope, not this one's. Nothing calls
stadium_density() yet either.

Verified: three-architecture boot (amd64, aarch64, riscv64), all
reaching ok> with identical dict_hash=0x3d4e1daf289da94f matching the
item-3.3 baseline.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

FABRIC.md

@@ -2773,7 +2773,29 @@ document and committing that amendment as its own item.*
   > `dict_hash=0x3d4e1daf289da94f`, matching the item-3.2 baseline — amd64
   > (`logs/20260804-170226`), aarch64 (`logs/20260804-170309`), riscv64
   > (`logs/20260804-170407`).
-- [ ] **3.4 — Density ranking.** Heat ÷ mass, read not computed. *Refs:* §19.2, §19.3.
+- [x] **3.4 — Density ranking.** Heat ÷ mass, read not computed. *Refs:* §19.2, §19.3.
+
+  > **DONE 2026-08-04.** `stadium_density(cell_index)` (`stadium.c`) reads a header's `heat`
+  > and `mass` and returns `heat / mass` — a division on demand from fields already stored
+  > in the cell, matching §19.3's "read, not computed by a scheduler" literally: no
+  > background process maintains this value. Stays valid Q48.16 without any special
+  > fixed-point routine, since `heat` is already Q48.16 and `mass` is a plain integer
+  > divisor.
+  >
+  > `mass == 0` and an out-of-range `cell_index` both return 0 rather than dividing by zero
+  > — an empty or never-admitted slot (everything is zero-initialized by item 3.2's
+  > `stadium_boot_init()`, and nothing yet births a patron into the Stadium) has no
+  > footprint to be dense within.
+  >
+  > **Deliberately not built here, per the item's own wording:** finding the densest or
+  > least-dense resident (§19.3's admission/eviction comparison) is item 3.5's scope — this
+  > function supplies the per-cell value that comparison will read, not the ranking/min-max
+  > machinery itself. Nothing calls `stadium_density()` yet either.
+  >
+  > **Regression: clean.** All three architectures boot to `ok>` with identical
+  > `dict_hash=0x3d4e1daf289da94f`, matching the item-3.3 baseline — amd64
+  > (`logs/20260804-171332`), aarch64 (`logs/20260804-171414`), riscv64
+  > (`logs/20260804-171513`).
 - [ ] **3.5 — Admission and eviction.** Admit if denser than the least dense resident.
   *Refs:* §19.3.
 - [ ] **3.6 — Hera as patron zero, pinned.** Assert at the eviction site; selecting Hera is

capsules/BLOCK_MAP.md

@@ -1,5 +1,5 @@
 # Capsule Block Manifest — Auto-generated
-<!-- Generated by mkcapsule --manifest  2026-08-04T21:03:52Z -->
+<!-- Generated by mkcapsule --manifest  2026-08-04T21:14:57Z -->
 <!-- DO NOT EDIT — re-run mkcapsule --manifest to refresh.  -->
 <!-- Hand-written justifications and immutability notes live  -->
 <!-- in MANIFEST.md alongside this auto-generated index.      -->

experiments/bare_metal/runs/doe-aarch64-20260804-171414.csv

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experiments/bare_metal/runs/doe-amd64-20260804-171332.csv

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include/starkernel/vm/stadium.h

@@ -178,6 +178,29 @@ typedef enum {
  */
 void stadium_dispatch(size_t cell_index, StadiumBehaviour behaviour);
 
+/*
+ * stadium_density - Heat / mass for the patron header at cell_index (FABRIC.md
+ * §19.2, §19.3). Read, not computed by a scheduler: both operands already
+ * live in the header, so this is a division on demand, not maintained
+ * bookkeeping. Result stays valid Q48.16, since heat is already Q48.16 and
+ * mass is a plain integer divisor.
+ *
+ * Returns 0 if mass is 0 -- an empty or never-admitted slot (everything is
+ * zero-initialized by stadium_boot_init() until something is actually born
+ * into the Stadium, which nothing yet does) has no footprint to be dense
+ * within, rather than a division by zero.
+ *
+ * Does not validate that cell_index actually holds a header rather than a
+ * continuation cell or an out-of-range index -- callers are expected to
+ * consult the item-3.1 discriminator bitmap first. Ranking (finding the
+ * densest or least-dense resident) is item 3.5's scope, not this one's;
+ * this function only supplies the per-cell value that comparison reads.
+ *
+ * @param cell_index  Index into the Stadium of the patron header to measure.
+ * @return Density in Q48.16, or 0 if the header's mass is 0.
+ */
+uint64_t stadium_density(size_t cell_index);
+
 #endif /* __STARKERNEL__ */
 
 #endif /* STARKERNEL_VM_STADIUM_H */

src/starkernel/vm/stadium.c

@@ -150,4 +150,23 @@ void stadium_dispatch(size_t cell_index, StadiumBehaviour behaviour) {
     }
 }
 
+/*
+ * FABRIC.md §19.2/§19.3: density is heat / mass, read on demand from fields
+ * already in the header -- not a value a scheduler maintains. mass == 0
+ * (an empty or never-admitted slot; everything is zero-initialized until
+ * something is actually born into the Stadium, which nothing yet does)
+ * returns 0 rather than dividing by zero. An out-of-range cell_index also
+ * returns 0 -- there is no patron there to be dense.
+ */
+uint64_t stadium_density(size_t cell_index) {
+    StadiumPatronHeader *header;
+
+    if (cell_index >= stadium_ncells) return 0;
+
+    header = &stadium_cell_array[cell_index].header;
+    if (header->mass == 0) return 0;
+
+    return header->heat / (uint64_t)header->mass;
+}
+
 #endif /* __STARKERNEL__ */